In recent years, as a mechanism for assisting operation of a steering wheel of a vehicle, an electric power steering (EPS) system that electrically generates torque is used. In the EPS system, unlike a hydraulic system, only when a driver's steering operation is performed, the vehicle steering operation is assisted. Therefore, the EPS system provides many advantages such as low fuel consumption.
As a motor serving as a torque generation source of the EPS system, a brushless motor that is driven to rotate by applying, for example, a three-phase alternating current is used. When such a brushless motor is used, it is necessary to produce an alternating current (AC) output, which is out of phase with a direct current (DC) output of a predetermined voltage (for example, 12V), so as to supply winding currents, which are out of phase with one another, to coils of plural phases (for example, three phases). This necessitates an electronic control unit for switching the coil currents of the motor. The electronic control unit includes semiconductor modules that implement a switching function.
In a conventional EPS system drive apparatus, an electronic control unit is disposed near a motor. For example, semiconductor modules are disposed in an axial direction of the motor (patent documents No. 1 and No. 2) or disposed around a stator included in the motor (patent document No. 3).